1. Field of the Invention
The present invention relates to a metal gasket having a coating layer formed thereon.
2. Description of the Prior Art
Metal gaskets are generally used as a means to provide a seal between a cylinder head and a cylinder block of an engine. They are either of single-plate structure composed of a beaded plate alone or of multiple-plate structure composed of a beaded plate and a multiplicity of laminated stiffening plates. Some of them in practical use have a coating layer formed on the surface of the beaded plate or stiffening plate for improved seal between the cylinder head and the cylinder block.
The coating layer is formed from NBR, fluororubber, or the like by spraying or spreading. Coating in this manner involves difficulties in using more than one kind of coating material at one time or leaving some parts uncoated. Therefore, it is common practice to form the coating layer from a single material on the entire surface of the beaded plate or stiffening plate.
When installed between the cylinder head and cylinder block, the metal gasket also seals the water jackets formed outside the combustion chambers. This means that some part of the coating layer formed on the gasket is exposed to coolant at all times and hence it is subject to peeling. Peeled fragments of the coating layer circulate together with coolant, clogging the thermostat and radiator tubes and eventually lowering the engine performance. A means in practical use to overcome this disadvantage is to mechanically remove the part exposed to coolant passage prior to installation.
If the coating layer exposed to coolant passages is to be partly removed after the coating layer has been formed on the entire surface of the beaded plate and stiffening plate, the production of metal gaskets would need complex steps and take a long time. Moreover, the partial removal of the coating layer means a waste of the coating material, which leads to a high production cost.
The conventional metal gasket in which the entire surface of the beaded plate and stiffening plate are coated with a single material suffers the disadvantage of being incapable of providing the optimal sealing whether it is used near the combustion chamber to seal high-temperature exhaust gas and coolant or it is used near the oil passage to seal coolant and lubricant.